Definition:
The reactions in which the carbon skeleton of the molecule is rearranged to give a structural isomer of the original molecule are known as rearrangement reactions. These rearrangements can also be considered as a sequence of steps belonging to substitution, additions or elimination reactions. Usually in such reaction atoms or groups shift from one position to another within the molecule resulting in a new molecular structure.

Types of Rearrangement reactions 1. Rearrangement of electron-deficient systems

2. Rearrangement of electron-rich systems

3. Migration of double and triple bonds

4. Thermal rearrangements

5. Rearrangement to an aromatic nucleus

A classical example of Molecular Rearrangement reaction is shown by the five membered cyclic transition state of acetoxyl groups in solvolytic reaction of benzoyl ephedrine.

Rearrangement of electron-deficient systems - The term anchimetric assistance (Gr. anchi + merso; adjacent parts) is often used to describe the help given by a migrating group in expelling a leaving group. This help could be hidden or obvious. When it is hidden it is called neighbouring group effect but when it is initiated by a nucleophile, it is generally called a rearrangement reaction. Neighbouring group effects are usually revealed by a special kind of stereochemistry or by an usually fast rate of reaction. Underlying most enzyme activity, are what amounts to neighbouring group effects, where reactants are brought together into exactly the right position to occur. The rearranging entity may be either a cation or a neutral molecule. Generally, the starting material forms an open ion, which goes to bridged ion, which in turn give rearranged open ion and later produce the product.

A B C D E

Mechanism in which an A B stage is absent is said to involve neighbouring group effect in ionization because the migrating group forms a bond in the transition state in which the bond to the leaving group is broken. The group that moves most easily are those that can support a positive charge in the transition state, i.e., electron rich groups and usually they have an electron pair to share.

There are two mechanisms, each beginning with protonation of the oxygen and each involving migration.

In the other pathway, there is an epoxide intermediate and the migration is in the same direction.

Example: Pinacol-pinacolone rearrangement occurs when vicinyl alcohols are dehydrated to give highly branched ketones. The product formed in unsymmetrical vincinyl alcohol is determined (a) by which OH group is first lost and (b) the group that migrates to the electron deficient carbon.

Rearrangement of Electron Rich Systems - These groups of rearrangement are the electronic counterpart of those treated in the last section. They are usually initiated by basic reagents that are able to remove a group or an atom, such as hydrogen. The residual anion then stabilizes itself by rearrangement.

Example: The Benzil-Benzilic Acid Rearrangement: The base-catalyzed rearrangement of 1,2-diketones takes its name from the most common example of this type of reaction. The driving force is provided by the addition of hydroxide ions to one of the carbonyl groups and by the ultimate formation of a salt from a strong base.

Allylic Rearrangements (Migration of Double and Tripple Bonds) - Substitution reactions at allylic positions often involve migration of double bond from its original position in the carbon skeleton to an adjacent site. There are four possible mechanistic possibilities for rearrangements reminiscent of substitution reaction.

Example 1: Rearrangement by SN1 mechanism.

Example 2: Rearrangement by SN2' mechanism.

Example 3: Rearrangement by SE1 mechanism.

Example 4: Rearrangement by SE2' mechanism.

Rearrangement to an Aromatic Nucleus - In many reactions, a group A migrates from a substituted hetero atom Z and becomes directly attached to an ortho or para position of an aromatic nucleus. The reaction is often acid-catalyzed.

Example: The Claisen rearrangement of aryl allyl ethers to allylphenols. If there is a vacant ortho position, only ortho rearrangement occurs. However if both ortho positions are blocked, rearrangements to an open para position take place.

Thermal Rearrangements - Many reactions involving high temperature often go by formation of radicals and ions but rearrangement reactions have been carried out in the vapour phase, so it is unlikely that the reaction involves dissociation of the starting material into fragments such as radicals or ions. Bonds are probably made and broken in the same transition state. The Cope rearrangement of doubly unsaturated systems of the bis-allylic type is the best known example.